A method of printing multiple colors by synthesizing colors on a recording medium (paper) is known, for example, as described in Japanese Patent Laid-Open Publication No. 3-244550.
According to this publication, there are provided a plurality of nozzle holes comprising first, second and third groups of nozzle holes respectively provided on the nozzle surfaces of ink jet heads for discharging magenta, cyan and yellow (hereinafter abbreviated as M, C and Y) ink, group by group. The multiple colors are printed by mounting the ink jet heads on a carriage and scanning in main and auxiliary scanning direction along the recording medium, and discharging ink from the nozzle holes.
In such a method, it is possible to synthesize printed dots of synthesized red, green and blue (hereinafter abbreviated as R, G and B) by precisely superimposing the printed dots of different primary colors (any of M, C and Y) over one another while shifting location by one pitch, or by integral times of one pitch, in the main or auxiliary scanning direction.
That is, it is possible to print synthesized multiple colors having no smudge therein with a relatively simple mechanism while performing a scanning operation which is not so different from that when printing monochrome. Accordingly, the multiple color printer using this method has quickly become widespread.
There are the following three kinds of methods employed in the aforementioned printing method.
First Method
This is a method, as shown in FIGS. 17 and 18, for making a print using printing heads on which groups of nozzle holes of different colors of M, C and Y are sequentially arranged in the auxiliary scanning direction.
This method comprises synthesizing colors by performing the main scanning while shifting every n pitch corresponding to respective groups of nozzle holes in the auxiliary scanning direction so that printed dots of different primary colors are superimposed over one another. Hereupon, the sequence of superimposition of the primary colors is always constant regardless of the normal or reverse direction of the main scanning direction (which is the same as the order of arrangement of the groups of different colors in the auxiliary scanning direction).
Accordingly, the tone of the same color in the printed image is always uniform even if the printing speed increases with bidirectional printing scanning. However, this is contrary to the demand for miniaturization and compactness of the apparatus because the dimensions of the heads in the auxiliary scanning direction increase, and hence the volume needed for the entire apparatus increases.
Second Method
This is a method, as shown in FIG. 19, for making a print using printing heads on which groups of n nozzle holes are provided for each different color of M, C and Y, and are sequentially arranged in the main scanning direction.
According to this method, the dimensions of the heads in the auxiliary scanning direction decrease, and hence the volume needed for the entire apparatus decreases. However, when printing, printed dots of different primary colors are superimposed over one another by n lines (width of n dots) under one-time main scanning which is not accompanied by scanning in the auxiliary scanning direction. Further, printed dots of different primary colors are superimposed over one another by n subsequent lines (by the lines extending from the n+1 line to the 2n line from the beginning) by performing scanning in the main scanning direction while shifting every next n pitch.
At this time, the sequence of superimposition of primary colors is reversed by reversing the main scanning direction, thereby changing the tone of the synthesized colors. Accordingly, when bidirectional printing scanning is performed, streaks are produced on the printed image every n pitch distance in the auxiliary scanning direction owing to the change of tone, thereby deteriorating the quality of the image.
Third Method
This is a method, as shown in FIG. 20, for making a print using printing heads on which groups of nozzle holes of different colors of M, C and Y are sequentially arranged in the main scanning direction while they are shifted by every one pitch (P) of printed dots in the auxiliary scanning direction.
In this case, the interval between respective groups of nozzle holes is three times as large as the pitch of the printed dots. Accordingly, the heads are easily manufactured. However, it is impossible to superimpose printed dots over one another by one time main scanning. Accordingly, it is necessary to superimpose printed dots of colors over one another by performing the main scanning while shifting the printed dots every one pitch (1/3 of the interval between nozzle holes of each group) in the auxiliary scanning direction.
Accordingly, in the first main scanning performed in first step ST1 as shown in FIG. 20, ink is prohibited from being discharged through a first nozzle hole M1 of an M group and a first nozzle hole C1 of a C group, while it can be discharged through other nozzle holes.
In FIG. 20, hatched portions represent colors to be printed in respective steps. Each dot represents that the printing is sequentially performed from the color positioned to its left.
Subsequently,in the second main scanning performed in second step ST2 after shifting by one pitch in the auxiliary scanning direction (sheet feeding by one pitch in a sheet feeding direction as denoted by the open arrow in the same figure), ink is prohibited from being discharged through the first nozzle hole M1 of the M group and ink can be discharged through other nozzle holes, if need be.
Next, in the third main scanning performed in third step ST3 after shifting further by one pitch in the auxiliary scanning direction (sheet feeding by one pitch), ink can be discharged through all nozzle holes.
Even in the fourth main scanning performed in fourth step ST4 after further shifting by (n-1).times.3+1 pitches in the auxiliary scanning direction, ink can be discharged through all nozzle holes. Further, even in the fifth main scanning performed in fifth step ST5 after shifting by one pitch in the auxiliary scanning direction, ink can be discharged through all nozzle holes.
Even in the sixth main scanning performed in sixth step ST6 after shifting by one pitch in the auxiliary scanning direction, ink can be discharged through all nozzle holes. The sequence of colors of printed dots in each line where the printed dots are superimposed over one another by the main scanning in sixth step ST6 follows the order: YCM, MYC, CMY, YCM, MYC, . . . YCM, YC, Y from the leading line, which is the same state as shown in third step ST3.
Accordingly, in the main scanning, performed in seventh step ST7 after shifting by one pitch in the auxiliary scanning direction upon completion of printing, ink can be discharged only through the last (nth) nozzle hole Mn of the M group and the last nozzle hole Cn of the C group.
In eighth step ST8, after shifting by one pitch in the auxiliary scanning direction, ink can be discharged only through the last nozzle Mn of the M group to complete the printing.
That is, the print is made from the start of printing through the steps of ST1, ST2, ST3, ST4, ST5, ST6, ST4, ST5, ST6, . . . ST4, ST5, ST6, ST7 and ST8. At this time, the sequence of superimposition of the printed dots of colors as a whole is as follows from the leading line, namely: YCM, MYC, CMY, YCM, MYC, . . . CMY, YCM, YCM, YCM, YCM, MYC, CMY, YCM, MYC, . . . CMY, YCM, YCM, YCM, MYC, CMY, YCM, MYC, . . . CMY, YCM, YCM and YCM.
Regarding the sequence of Y and C according to the sequence of colors as set forth above, it is changed in some lines such as YC, YC, CY, YC, . . . . Even in the order of C and Y and M and Y, the reverse sequence of colors occurs likewise in some lines.
If the reverse of the sequence of colors occurs, R, G and B which are synthesized colors of each line are changed in tone, thereby producing streaks that deteriorate the quality of the image.
The sequence of superimposition of the printed dots of colors is always fixed regardless of the forward or reverse direction of the main scanning direction. Accordingly, such streaks can not be removed even if the main scanning direction is selected in either direction (i.e., even in the normal printing method or in the bidirectional printing scanning method).
It is, therefore, an object of the invention to remove streaks in printed images which are produced by the change of sequence of the superimposition of colors using a color printer provided with the ink jet heads which are used in the aforementioned third method.
It is another object of the invention to provide a small-sized color printer capable of solving the above problem and using ink jet heads which are easily manufactured, and a printing method capable of printing multiple colors which are uniform in image quality even if one-way printing or bidirectional printing is performed.